1. Field of the Invention
The present invention generally relates to a linear motion guide unit, and, in particular, to a finite linear motion rolling contact guide unit which is enclosed in a shell.
2. Description of the Prior Art
A finite linear motion rolling contact guide unit is well known in the art, and its typical prior art example is shown in FIGS. 5 through 7 and another typical prior art example is shown in FIG. 8. A finite linear motion rolling contact guide unit generally includes a rail, a table slidably mounted on the rail, a plurality of rolling members interposed between the rail and the table and a retainer for retaining the rolling members in position. With this structure, a relative linear motion can be provided between the rail and the table over a predetermined distance.
The linear motion guide unit shown in FIGS. 5 through 7 include a rail 21 elongated and generally rectangular in shape and typically comprised of aluminum and a table 22 which is generally U-shaped in cross section, including a horizonal section and a pair of vertical sections depending from the opposite sides of the horizonal section, and typically comprised of aluminum. As best shown in FIG. 6, an inner guide groove generally rectangular in cross section is formed at each side surface of the rail 21 and an outer guide groove generally rectangular in cross section is also formed at each side surface of each of the vertical sections of the table 22 in an opposed relationship with its associated inner guide groove to thereby define a guide channel 23 extending in parallel with the longitudinal axis of the guide unit. Thus, the guide unit is provided with a pair of guide channels 23 on the opposite sides of the rail 21, and each of the guide channels 23 has a generally rectangular cross section.
In each of the guide channels 23 are provided with a plurality of rolling members (balls in the illustrated example) 25 and also with a plate-shaped retainer 24 formed with a plurality of holes spaced apart from each other for retaining therein the balls 25 in a specific spaced apart relationship. Also provided in each of the guide channels 23 are four guide steel wires 26, two of which are located at the top and bottom corners of the inner guide groove and the remaining two of which are located at the top and bottom corners of the associated outer guide groove. Thus, as best shown in FIG. 6, the balls 25 are in rolling contact with these guide wires 26 so that a rolling contact is provided between the rail 21 and the table 22.
Also provided in the prior art guide unit shown in FIGS. 5 through 7 is a preload adjusting mechanism 27 which generally includes a pressure plate and associated screws which may be turned to move inward or outward to adjust the location of the pressure plate to thereby adjust the preloading state of the balls 25.
The above-described prior art is disadvantageous in many respect, though the rail 21 and the table 22 themselves are relatively rigid in structure because they are made from solid blocks by machining them. For example, since the rolling contact is provided between the balls 25 and the wires 26, its load bearing capability is rather limited. In addition, since the number of elements is relatively large, it is rather complicated and thus expensive to manufacture.
FIG. 8 illustrates another typical prior art linear motion guide unit. As shown, this prior art guide unit includes a generally U-shaped rail 28, a generally U-shaped table 29, a plurality of rolling members or balls 31 and a retainer 30 for retaining the balls 31 in position. In this guide unit, both of the rail 28 and the table 29 are fabricated from a relatively thin sheet metal, such as a steel plate, by bending into a predetermined shape having a generally U-shaped cross section, and the table 29 thus fabricated is turned upside down and placed onto the rail 28 which is held right side up with its open top side facing upward.
Also in this guide unit, an inner guide groove is formed in each of the outer side surfaces of the rail 28 and an outer guide groove is formed in each of the inner side surfaces of the table 29 in an opposed relationship to thereby define a guide channel in which the balls 31 and the retainer 30 are disposed. In this case, the balls 31 are in rolling contact with both of the inner and outer guide grooves to provide a rolling contact between the rail 28 and the table 29. Such a structure is possible in this guide unit since the guide grooves can be defined relatively at high accuracy for example by press forming at the time when the rail 28 and the table 29 are fabricated.
In the above-described second prior art guide unit, since the balls 31 are in rolling contact with the inner and outer guide grooves directly, a relatively large load bearing capability can be provided. In addition, since both of the rail 28 and the table 29 are fabricated from a thin metal plate, for example, by bending or press forming, they can be fabricated relatively easily and at low cost. In addition, the overall size and weight of the guide unit can be made much smaller. However, since both of the rail 28 and the table 19 are made from a thin metal plate, they are subject to distortions so that they present limitations in the load bearing capability. Moreover, since the rail 28 and the table 29 are relatively thin, mounting holes provided in them are very shallow so that difficulty is often encountered in mounting the rail 28 and the table 29 to appropriate objects, such as a frame or a moving component of an apparatus.